Lead zirconate titanate that contains lead is a piezoelectric material most widely used in various piezoelectric devices. Attempts have been made to replace lead-containing piezoelectric materials by lead-free piezoelectric materials. This is to address the concern that lead-containing piezoelectric devices once discarded and exposed to acid rain may inflict damage on ecosystem as the lead component in the piezoelectric materials may leach into earth. Thus, various proposals of piezoelectric material have been made.
One example of the lead-free piezoelectric material is a tungsten bronze structure material containing, for example, barium bismuth niobate as a main component. PTL 1 discloses a material system that contains barium lithium niobate as a main component and bismuth niobate as an accessory component. However, since the unit cell of crystals having a tungsten bronze structure has a high shape anisotropy, the polar axis direction lies only in the c-axis direction, i.e., the short side direction of the unit cell. Accordingly, there have been fewer effective domains that can contribute to piezoelectricity and the piezoelectric properties have been unsatisfactory.
Another approach to enhancing the properties of a lead-free piezoelectric material having a tungsten bronze structure is to orient the piezoelectric material by using a magnetic field (magnetic orientation). Magnetic orientation can increase the number of effective magnetic domains that contribute to piezoelectricity. PTL 2 discloses a technique of using anisotropic particles as the starting material to improve the degree of orientation in conducting magnetic orientation. However, this technique is applicable only to piezoelectric materials that have a composition that can use anisotropic particles as the starting material. Thus, it has been difficult to obtain an oriented barium bismuth niobate-based material having good properties.
The present invention provide a lead-free piezoelectric material including a barium bismuth niobate-based tungsten bronze structure metal oxide having a high degree of orientation. A piezoelectric element, a liquid discharge head, an ultrasonic motor, and a dust cleaning device that include the piezoelectric material are also provided.